1. Field of the Invention
The present invention relates to reducing the color forming tendencies of certain esters and, more particularly, to the treatment of esters with an alkali metal borohydride in the presence of water.
2. Description of the Prior Art
Borohydrides are recognized as compounds for the selective reduction of a wide range of functional groups, e.g., aldehydes, ketones, imines, esters and amides, etc. The use of borohydrides is compiled in the publication entitled Process Stream Purification, Bulletin No. BA-8UG, Ventron Corp., Sept., 1978. This publication lists 6 patents relating to the purification of esters, but it does not include any relating to the removal of color formers from aliphatic dibasic esters.
U.S. Pat. No. 2,957,023 issued on Oct. 18, 1960 discloses a method for the stabilization of alcohols contaminated with carbonyl and/or unsaturated compounds such as olefins by adding a borohydride to the alcohol during or before the esterification. The patentees disclose that a solvent is preferred because of the relative insolubility of the borohydride but indicate that the choice of solvent is not critical to their invention. Solvents which are disclosed include water, isopropanol ether, low molecular weight amines and the like.
The production of color free polyvinyl alcohol is obtained according to the teachings of U.S. Pat. No. 3,679,646 issued on July 25, 1972 by pre-treating an alcohol solution of a polyvinyl ester with from about 0.03 to about 3% by weight based upon the weight of the polyvinyl ester of an alkali metal borohydride followed by alkaline hydrolysis. The patentees prefer to use the borohydride in a finely divided, e.g., powder form.
U.S. Pat. No. 3,681,482 issued on Aug. 1, 1972 discloses a process for improving the color of phosphate esters by contacting the esters with 0.002 to 0.05 percent by weight of sodium or lithium borohydride. The patentees disclose that the borohydride is preferably in solid form but can be in aqueous solution.
German Pat. No. 2,556,258 issued on May 26, 1977 teaches the stabilization of polyhydroxy carboxylate polymers by the addition of sodium borohydride.
U.S. Pat. No. 3,991,100 issued on Nov. 9, 1976 discloses a process for the production of esters of dibasic acids which exhibit a reduced color-forming tendency under alkaline or transesterification conditions. The process involves heating the esters to remove water, oxides of nitrogen and nitric acid, esterifying the thus heated product and finally distilling off volatile materials in the presence of a dry base such as sodium carbonate and calcium hydroxide. It should be noted, however, that this method does not completely eliminate the color-forming tendencies of the esters during transesterification.
The art discloses that under ordinary conditions aliphatic nitro compounds are not reactive with sodium borohydride and that for such a reaction to occur to any significant extent transition metals, their salts and complexes of Lewis acids are required (Sodium Borohydride, Ventron Corporation, Dec., 1979 pages 35-36).